Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing the new design.

The broken lines showing a display screen or portion thereof, and all other broken lines showing portions of the graphical user interface, form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 